A variety of stain-resistant coating agents have long been available commercially, primarily for residential construction materials such as glass and exterior walls.
Previously, techniques in which water repellency was used to repel the actual contaminants were widely employed. However, in recent years, it has become increasingly obvious that as a result of long-term exposure of these types of water-repellent films, a problem has arisen in that the water repellency of the material has become a source of electrostatic charging, which actually promotes the adsorption of dust and the adhesion of soiling.
In light of these circumstances, a variety of hydrophilic coating solutions have begun to be tested, with the expectation of preventing electrostatic charging and enabling soiling to be washed away by rainfall, and various methods of hydrophilizing members are being investigated, mainly focused on photocatalytic materials. In particular, photocatalysts are not only hydrophilic upon irradiation with light, but can also be expected to exhibit an oxidative decomposition action on soiling, and there are many practical examples of such photocatalysts.
For example, applications to organic materials including plastic materials and applications to decorative materials have been actively investigated in recent years, mainly in relation to applications to exterior tiles, glass, exterior wall coatings, internal filters of air cleaners, and inorganic substrates (such as ceramics and metals) (see Patent Document 1 and Patent Document 2).
However, currently, all of the hydrophilic coating solutions that have been marketed and developed in the manner described above using numerous components suffer from some form of problem.
Specifically, hydrophilic coating agents containing a water-soluble polymer as the main component have poor durability, and as the curing reaction of the coating film progresses, the hydrophilic active groups such as hydroxyl groups are gradually eliminated.
Inorganic coating agents such as silicones exhibit excellent durability and hydrophilicity, but the coating solution itself is often an organic solvent-based system, which can sometimes limit the working conditions. Even solventless systems suffer from small amounts of volatilization of volatile oligomers, meaning the working conditions are limited. Particularly in the case of coating agents having high curing reactivity, storage problems also arise due to the short lifespan of the solution.
In coating solutions containing an added photocatalyst, because the photocatalyst itself is a powder having extremely high cohesion, the coating solution must be formed as a coarsely dispersed sol (dispersed particle size: 100 nm to several μn) to which any of the various polymer vehicles has been added, and therefore the external appearance following coating may sometimes develop cloudiness, resulting in inferior transparency. Further, the hydrophilicity of the coating film can only manifest in environments where light is present. Moreover, decomposition and degradative dislodgement of the coating film caused by decomposition of the vehicle by photocatalytic reactions is also unavoidable.
In this manner, there is no coating solution currently available which is safe and easy to work with, and can form a film capable of exhibiting a satisfactory hydrophilic action over a long period of time.
Further, these coating agents often contain an added metal oxide such as silica containing a large number of hydroxyl groups in order to impart good hydrophilicity, but particles of these metal oxides have a high refractive index, and when coated onto a transparent substrate, and particularly a substrate having a low refractive index such as glass or polycarbonate, tend to display high reflectance relative to diagonally incident light striking the coated surface at an angle other than the normal direction, meaning the external appearance is often impaired.
Accordingly, there is considerable demand for a technique capable of simultaneously addressing the problems facing existing hydrophilic coating solutions, namely problems such as:    (1) difficulty in achieving high hydrophilicity,    (2) inability to maintain hydrophilicity for long periods,    (3) low transparency for the solution itself or the formed coating film, meaning application to substrates other than white substrates is difficult,    (4) limits on the pot life and/or the coating properties of the solution itself, and    (5) severe reflection from the coating film, thus impairing the external appearance, especially when viewed from an angle.
[Patent Document 1] JP 2006-116461 A
[Patent Document 2] JP 2006-272757 A